Field of the Invention
The present invention relates to a secondary battery capacity measuring system and a secondary battery capacity measuring method for measuring the maximum capacity of a secondary battery.
Priority is claimed on Japanese Patent Application No. 2016-009902, filed Jan. 21, 2016, the content of which is incorporated herein by reference.
Description of Related Art
All patents, patent applications, patent publications, scientific articles, and the like, which will hereinafter be cited or identified in the present application, will hereby be incorporated by reference in their entirety in order to describe more fully the state of the art to which the present invention pertains.
A secondary battery which is rechargeable is used as a power source for a running motor of a hybrid automobile, an electric automobile, or the like, and is also widely used in the industrial field, public institutions, ordinary households, and the like from the viewpoint of accumulating energy with a relatively small environmental load such as through solar power generation or wind power generation without depending on fossil fuel.
In general, such a secondary battery is configured as a battery module in which a predetermined number of battery cells are connected in series to acquire a desired output voltage, or is configured as a battery pack in which a predetermined number of battery modules providing a desired output voltage are connected in parallel to acquire a desired current capacity (Ah).
A maximum capacity of a battery is used as an index indicating performance of a secondary battery. The maximum capacity can be defined as a current capacity (Ah) which is output until the open-circuit voltage of the battery decreases from the maximum voltage to the available minimum voltage in the available voltage range of the battery.
Whenever the secondary battery is repeatedly charged and discharged, characteristics thereof are deteriorated and the maximum capacity is gradually decreased due to deactivation or side reaction of active materials responsible for electrical conduction. The active materials are materials of a positive electrode and a negative electrode. For example, in a Li (lithium) ion battery, a carbon material is used for the negative electrode and a lithium transition metal oxide is used for the positive electrode.
In the secondary battery, energy of the active materials is biased in areas close to a lower limit and an upper limit of a state of charge (SOC), which is a ratio of a remaining capacity to the maximum capacity of the battery, toward an overcharged or overdischarged state, whereby deterioration thereof rapidly progresses.
Accordingly, in the secondary battery, it is necessary to restrict charging and discharging in areas close to the maximum voltage and the minimum voltage. By operating the secondary battery in a predetermined SOC range, it is possible to suppress deterioration and to extend a life span of the secondary battery in comparison with a case in which overcharging or overdischarging is carried out. Here, the predetermined SOC range is generally in a range of 50%±30%, that is, about 20% to 80%, of the maximum capacity.
In order to manage and operate the secondary battery in the predetermined SOC range, it is important to accurately understand the SOC. The SOC can be acquired by integrating a charging/discharging current at a time of charging or discharging of the battery. However, when the SOC is acquired using the charging/discharging current, an error in analog/digital (A/D) conversion of a current sensor is accumulated over long-term operation of the battery. In order to correct the error, it is necessary to perform an SOC calibrating operation at a certain time. Since the SOC is defined as a ratio of a remaining capacity to the maximum capacity, it is essential to accurately understand the maximum capacity of the secondary battery during deterioration in order to accurately understand the SOC.
The maximum capacity is generally acquired by time integration of a minute current (a minute discharging current) while allowing the secondary battery to completely discharge through the minute current after fully charging the secondary battery.
Accordingly, for example, in a stationary electricity storage system, an SOC temporarily departs from the SOC range in a normal operation mode and an operation mode is switched from the normal operation mode to an evaluation mode in which the maximum capacity is measured for a long time.
Here, as a time associated with the evaluation mode increases, a time in which operation of the secondary battery stops is increased and operational efficiency of the secondary battery is reduced.
Accordingly, when acquisition of the maximum capacity of the secondary battery is intended, it is necessary to estimate the maximum capacity of a deteriorated battery for a short time at a low cost without departing from the SOC range in the normal operation mode.
For this reason, a technique of estimating a maximum capacity of a secondary battery using charging and discharging characteristics in some sections of an estimation subject secondary battery and using a correlation between characteristics in the sections and the maximum capacity when estimating the maximum capacity of the secondary battery is known (for example, see Japanese Unexamined Patent Application, First Publication No. 2014-002122, Japanese Unexamined Patent Application, First Publication No. 2009-252381, and “A unified open-circuit-voltage model of lithium-ion batteries for state-of-charge estimation and state-of-health monitoring”, Caihao Weng et. al, journal of Power Sources 258 (2014), p. 228-237). Accordingly, it is possible to shorten the time required for charging and discharging for estimating the maximum capacity of the secondary battery using characteristics in some sections narrower than an SOC range from 0% to 100% which is the whole section.
In Japanese Unexamined Patent Application, First Publication No. 2014-002122, a maximum capacity in discharging is measured using a linear correlation between a capacity value and a maximum capacity when an inter-terminal voltage of power supply terminals of a secondary battery is discharged from a predetermined first voltage to a second voltage.
In Japanese Unexamined Patent Application, First Publication No. 2014-002122, since the linear correlation improves as a range of a discharging area used to estimate the maximum capacity is increased, the estimation accuracy of the maximum capacity is improved and thus discharging is performed in an SOC range corresponding to 10% to 90%.
In Japanese Unexamined Patent Application, First Publication No. 2009-252381, the maximum capacity of a secondary battery is estimated using differential characteristics “dV/dQ vs Q” which are observed in an area in which the voltage is flat in charging/discharging characteristics (an area in which an SOC ranges from 10% to 90%). Here, a maximum capacity value in charging is estimated using a linear correlation between a distance (ΔQ) between two feature points (maximum values) in the differential characteristics and the maximum capacity. The distance ΔQ represents the difference in capacity value Q between two maximum values of dV/dQ.
In “A unified open-circuit-voltage model of lithium-ion batteries for state-of-charge estimation and state-of-health monitoring”, Caihao Weng et. al, journal of Power Sources 258 (2014), p. 228-237, charging/discharging characteristics are expressed using a function model, curve fitting of the function model is performed on measured values in an area corresponding to an SOC range of 10% to 90%, and parameters of the function model are identified. Then, feature points (maximum values) of differential characteristics “dQ/dV vs V” are acquired from the function model of which the parameters are identified. The maximum capacity at that time is estimated using a linear correlation between the acquired feature points and the maximum capacity.
In the above-mentioned related art, for example, measured data of charging or discharging in a broad section which is in the SOC range of 10% to 90%, but not in the whole SOC range, is required for acquiring the feature points necessary for estimation. When a secondary battery of which a maximum capacity is to be estimated is a single cell, a charging process can be performed in the SOC range of 0% to 100% and thus the feature points can be acquired in the broad range of 10% to 90% without any problems.
However, when the secondary battery of which the maximum capacity is to be estimated is a combination battery (a battery module) in which a plurality of cells are connected in series, degrees of deterioration of combined cells are different, and thus it is not possible to estimate the maximum capacity in the above-mentioned related art.
That is, when at least one cell having a very large degree of deterioration is mixed with cells of a secondary battery, the cell having a large degree of deterioration has a maximum capacity lower than that of the other cells having a small degree of deterioration. Accordingly, the inter-terminal voltage of the cell having a large degree of deterioration reaches an upper-limit voltage earlier than the cells having a small degree of deterioration. As a result, a charging process on all the cells including the cells having a small degree of deterioration is stopped due to the stopping of the cell having a large degree of deterioration. Accordingly, the charging process on the secondary battery ends in a state in which charging of the cells having a small degree of deterioration is not completely performed.
Accordingly, in the cells other than the cell having a large degree of deterioration, measured values in an SOC range required for acquiring feature points to be used to estimate the maximum capacity cannot be acquired. Accordingly, since the feature points to be used to estimate the maximum capacity cannot be acquired, it is not possible to estimate maximum capacities of the cells other than the cell having a large degree of deterioration.
On the other hand, from the viewpoint of efficient operation and safety of a battery management system (BMS), it is very important to understand maximum capacities of all cells in a secondary battery. Recently, a combination battery in which cells having different degrees of deterioration are combined is often used as a secondary battery. If maximum capacities of the cells having different degrees of deterioration cannot be estimated, it causes a severe problem in operation of the BMS.
In Japanese Unexamined Patent Application, First Publication No. 2014-002122, a maximum capacity is estimated on the premise that a capacity value between a first feature point and a second feature point has a linear correlation with the maximum capacity. That is, Japanese Unexamined Patent Application, First Publication No. 2014-002122 is based on the premise that a deterioration rate of the maximum capacity is the same in any capacity area.
In general, it is known that a peak in a curve of differential characteristics “dQ/dV vs V” appears when a crystal structure (a stage structure) of an active material, such as graphite which is widely used in a lithium ion battery, varies. When the crystal structure varies, Li ions are inserted into graphite. Here, an area obtained by performing integration between the feature points in the curve of differential characteristics “dQ/dV vs V” corresponds to the amount of charge Q inserted into graphite.
That is, in view of the curve of differential characteristics “dQ/dV vs V,” the premise that the deterioration rate of the maximum capacity is the same in any capacity area corresponds to a decrease of dQ/dV on a vertical axis at the same rate.
A technique of estimating a battery state such as a maximum capacity or amounts of active materials of a positive electrode and a negative electrode by fitting them to measured values of some sections using a variable indicating a decrease rate of the maximum capacity on the basis of the premise that the maximum capacity decreases at the same rate in any process of variation in a crystal structure of graphite is also known (for example, see Japanese Unexamined Patent Application, First Publication No. 2009-080093).
However, it has also been reported that the variation rate of the maximum capacity in variation of crystal structures actually varies (“Aging of a Commercial Graphite/LiFePO4 Cell”, M. Safari et. al, Journal of the Electrochemical Society 158 (10) A1123-A1135 (2011)), and thus the decrease rate of the maximum capacity in the variation of the crystal structure (feature points) is not necessarily the same as described in Japanese Unexamined Patent Application, First Publication Nos. 2014-002122, 2009-252381, and 2009-080093. That is, partially due to an influence of deterioration of a positive electrode in a lithium ion battery, an amount of lithium ions Li varies depending on variation areas (areas between the feature points) of a crystal structure of a graphite negative electrode. For example, the amount of Li ions, such as LiC6, LiC12, LiC27, and LiC36, inserted into the graphite negative electrode varies depending on the crystal structure. It is also considered that an influence level of the amount of Li ions to deterioration varies depending on the crystal structure, and the premise that the decrease rate of the maximum capacity in the variation of the crystal structure (feature points) are the same and a difference from an actual state in the lithium ion battery cause a capacity estimation error in estimation of the maximum capacity.
As described above, in the related art, measured values in a broad SOC range are necessary for acquiring feature points to be used to estimate a maximum capacity. Accordingly, when a cell of which the maximum capacity is deteriorated more than the other cells is present in a combination battery, there may be a cell of which the maximum capacity cannot be estimated.
A technique of estimating a maximum capacity by fitting it to measured values in some SOC ranges is also known. However, when the maximum capacity is estimated on the premise that an amount of charge Q charged in all processes of variation of a crystal structure decreases at the same rate due to deterioration of the secondary battery, a decrease rate of the amount of charge charged for every variation of the crystal structure may vary, which may be an error factor in the estimation of the maximum capacity.